Wire drawing is a process employed to produce wire from rod by pulling the rod and wire through one or more dies in order to reduce the cross-sectional area until a final product of the desired cross-section is achieved.
"Rod" is a term used to denote hot-rolled, undrawn stock used in the wire drawing process. "Wire" is the term used to denote the product of drawing, i.e., rod which has been reduced in cross-sectional area.
Dies used in the wire drawing process must be of sufficient hardness to withstand the pressure, heat, and abrasiveness developed by the wire passing through the die. Most wire drawing dies are constructed of special alloys such as tungsten-carbide or similar hard materials or alternatively, the die surfaces, which may contact the moving wire, are coated with thermally stable, abrasion resistant coatings. Direct contact between the die surface and the moving wire surface must be kept to a minimum, or preferably prevented entirely, in order to maintain the desired surface characteristics of the wire and prevent excessive die wear and damage.
Typical dies designed for wire drawing operations consist of four zones which may be described as follows: Zone 1, or the approach zone, consists of a circumferential angular opening encircling the moving wire which allows the wire drawing lubricant to enter the die. The angle of the approach zone's interior surface, relative to the moving rod or wire surface, is typically 6 degrees to 25 degrees. The selection of approach zone angle depends on the size and composition of the wire to be drawn, draw speed, number of reductions required, and lubricant formulation and physical form. The lubricant must be in a form which allows it to enter the approach zone along with the wire. Zone 2, or reduction zone, is the location within the die in which plastic deformation of the rod or wire occurs. It is in Zone 2 that reduction of cross-sectional area is achieved during drawing. Zone 2 is a continuous extension of Zone 1, encircling the moving wire. The angle of the interior surface of Zone 2 relative to the moving wire determines both the degree of cross-sectional reduction and is a major factor in controlling the thickness of the wire drawing lubricant film which remains on the wire surface as it exits the die. This residual lubricant is essential when a number of dies are used in a series to effect multi-step cross-sectional reductions. Zone 3 is referred to as the bearing zone. It serves principally to assure final shaping of the wire. Zone 4 is the pressure relief zone. Pressure developed between the wire and die surfaces can reach many thousands of pounds per square inch during the drawing operation. It is necessary that this pressure be released at the die exit in a manner which avoids damage to the die. Without a pressure relief zone, cracking of the die can occur.
Dies may be used in combination with a single die stand. These are referred to as pressure dies and are designed to increase the pressure on the wire drawing lubricant in order to force additional lubricant onto the surface of the wire and thus increase the residual lubricant film thickness.
As noted above, it is essential that the rod or wire be prevented from coming in contact with the die surface during wire drawing. This is accomplished by maintaining a continuous film of lubricant between the die surface and the surface of the moving wire. When dry wire drawing lubricants are used, the rod or wire is pulled continuously through a bed of dry wire drawing lubricant contained in a "soap box" or "die box." The soap box has an entry port and an exit port through which the wire passes. The exit port of the soap box is comprised of a first die located such that the die is below the surface level of the wire drawing compound contained in the soap box. Periodic additions of wire drawing compound are made to the soap box to assure that its first die is always submerged in wire drawing compound.
When a series of dies are employed for multi-step reductions, there may be additional soap boxes associated with specific dies. The purpose of these additional boxes is to supply additional surface lubricant coating to the wire if needed.
The wire being pulled through the die system travels at speeds of a few feet per minute, up to thousands of feet per minute, depending on the die system, wire composition, cross-sectional area reduction required, cooling capacity, and lubrication available. At these high speeds it is necessary that the undrawn rod surface be roughened so that lubricant in sufficient quantity will adhere to the surface and be carried into the die. Roughening of the rod may be accomplished by applying chemical coatings to the rod prior to its introduction into the wire drawing system. The most common coating compositions are based on lime, borax, or phosphates. The resultant rough coating is commonly referred to as a "lubricant carrier" coating.
Mechanically descaled rod may be sufficiently rough without further coating or, if necessary, may be roughened with additional mechanical treatment. Lubricant applicators can be used to force lubricant onto the rod surface by pressure.
The dry wire drawing compound lubricants must flow freely in the soap box in order that fresh lubricant be exposed to the moving wire. If the wire drawing compound fails to move freely by gravitational force or mechanical agitation in the soap box, it will compact into a dense mass through which the moving wire will form a channel. This is a condition known as "tunneling." Once tunneling occurs, there is a loss of contact between the wire and the dry lubricant and, as a result, the die system is starved for lubricant and damage to the wire and die surface will occur.
As the dry wire drawing compound lubricant enters the die at the approach zone, it is converted by heat and/or pressure into a film of plastic-like consistency. If converted to a liquid, it would offer little, if any, protection against the wire moving laterally through it and contacting the die surface. Further, the majority of a liquid lubricant applied to the wire in this type of drawing system would be lost immediately upon exiting the die and would not be available as residual lubricant for protection of other dies in a multi-die system.
The composition of the dry wire drawing compound lubricants has been discussed widely in the patent and technical literature, some examples of which are set forth hereinafter in the detailed description. In a broad sense, dry wire drawing compounds are typically based on a combination of fatty acid soaps, excess base or free fatty acid, and, as required for specific applications, various thickeners, pressure additives, pigments, fillers, and thermal stabilizers. The most commonly used dry wire drawing compound lubricants are based on calcium soaps or sodium soaps. A manufacturer of dry wire drawing compound lubricants typically offers several hundred different formulations, each designed to satisfy the technical requirements of specific wire drawing applications.
Historically, dry wire drawing compound lubricants have been produced as fine powders in order to meet the stringent requirements of the wire drawing process. However, these powdered materials are very dusty, lending to worker irritation and unclean work areas.
Various approaches have been tried to alleviate the dust problems associated with dry wire drawing compound lubricants. These include tableting, extruding, flaking, beading, and wetting. None, however, have been totally successful.
Wetting of the compound with a liquid to suppress dustiness introduces a non-active diluent which frequently has a deleterious effect on one or more essential properties of the lubricant, such as lowering of the melt point or reduction in free flowability.
"Beading" is a process of manufacturing dry wire drawing compound lubricants disclosed in Canadian Patent 1,006,497. Although this patent discloses a composition which is "essentially dust-free," it states that "the presence of fines in minor amounts . . . can be tolerated without loss of operating efficiency." In practice, these beaded compositions are less than completely dust free as would be expected from the presence of fine particles. Removal of the fines by screening or washing would add costly manufacturing steps. Further, the beads formed by rolling are not uniform in dimension in any direction, resulting in separation during shipment and use.
Flaking of dry wire drawing compound lubricants by casting a molten mass of the lubricant onto a chill roll is essentially ineffective. The resultant flakes are too large, typically one-half inch in diameter (12 mm), to perform effectively in wire drawing systems. Grinding of the flakes to produce smaller particle size invariably leads to production of a fine powder fraction and dust.
Tableting is an expensive process and, again, the particle size, typically one-quarter inch in diameter (6 mm) or greater, is unsatisfactory.
Extruding of dry wire drawing compounds on conventional screw extruders, operated in a conventional manner, such as are used in making pelletized plastics or plastic additives has been tried in the dry wire compound lubricant industry without success. While the pellets produced were dust free, the work energy required to form them hardened the pellets so that they would not melt or reduce to useful size in the wire drawing process.
It is completely surprising that the process of the instant invention solves all of the problems of previous attempts at making effective, dust free, dry wire drawing compound lubricants, especially since no permanent additional additives such as water-soluble binders which could interfere with or change the lubrication properties of the dry wire drawing compound lubricants are required.